How do I model the express key of a wacom tablet?
Add a Circle mesh and select half of the vertices.

Press Y to separate the selection, and move it away along whichever axis it faces.

Select all (A) and press F to fill the shape with an n-gon.

In Edge Select Mode (2), select the two longest edges, press W and select Subdivide.

Select the new edge and press ⎈ CtrlB (Bevel Tool). Scroll  MW up once or type 2 to get two segments.

Select everything and press E to extrude the shape.

Select the center edge and move it down.

Delete the faces on the bottom.
Finally, select all the sharp edges and bevel them to your liking.

I'd recommend shading smooth at this point. ;)